ELECTRODEPOSITION OF CdTe ON STAINLESS STEEL 304 ...

Electrodeposition of CdTe on Stainless Steel 304 substrates

By Patrick Rutto

Submitted in Partial Fulfillment of the Requirements

for the Degree of

Master of Science

in the

Chemistry

Program

YOUNGSTOWN STATE UNIVERSITY

May, 2018

Electrodeposition of CdTe on Stainless Steel 304 Substrates

Patrick Rutto

I hereby release this thesis to the public. I understand that this thesis will be made available from the OhioLINK ETD Center and the Maag Library Circulation Desk for public access. I also authorize the University or other individuals to make copies of this thesis as needed for scholarly research. Signature: Patrick Rutto, Student Date Approvals: Dr. Clovis A. Linkous, Thesis Advisor Date Dr. Timothy Wagner, Committee Member Date Dr. Tom N. Oder, Committee Member Date Dr. Salvatore A. Sanders, Dean of Graduate Studies Date

iii

ABSTRACT

The energy we get from the sun is a key factor in electric power production on earth and

in space applications. The development of photovoltaic cells has enabled a new direct

method for solar electricity. However, the manufacturing cost of photovoltaic cells must

be lowered to have widespread implementation. Among the leading candidates CdTe, is

used for photovoltaic applications, since it has optimum band gap energy for the efficient

conversion of solar energy into electricity. It is produced by a series of vacuum

procedures, which is a significant part of its fabrication cost. In this work, cadmium

telluride (CdTe) thin films were electrodeposited on stainless steel 304 substrates using a

three-electrode system at a negative potential. Cadmium sulfate and tellurium dioxide in

pH 1.8 H2SO4 were used as the cadmium and tellurium sources, respectively. Deposition

conditions were adjusted to codeposit Cd and Te at the same rate. Films were deposited

on stainless steel 304 as a relatively inexpensive substrate. However, to obtain a proper

ohmic contact between CdTe and the steel, it was necessary to electrodeposit a thin

interlayer of pure Te to achieve an ohmic contact. Electrodeposition of a thin Pt layer on

top of the CdTe served to greatly increase the rate of H2 evolution. The structural and

morphological properties of the resulting films were characterized using light/dark

voltammetric methods, X-ray diffraction (XRD), Profilometry, Scanning Electron

Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS).

iv

ACKNOWLEDGEMENT

I would like to acknowledge the help of several individuals who have assisted me in

various aspects of this thesis and the work relating to it. Foremost, I would like to express

my sincere gratitude to my advisor Dr. Clovis A. Linkous for the continuous support of

my study and research, for his patience, motivation, enthusiasm, and immense

knowledge. Thank you so much for supervising the research which has led up to this

document and for the countless candies which were provided through regular visits to his

office. I would also like to thank Dr. Oder for the time, supervision, and direction he has

given to the research behind this thesis as well as the thesis itself. Dr. Timothy Wagner

thanks for guiding me in many of the improvements that have been made to this

document. I would also like to thank Ray for his help with the instruments. Of course; I

would like to thank my parents for their support too. A special thanks to fellow students

Solita, Milica, Seon, Linda and Omweri without whom nothing practical would have

gotten done. I am deeply grateful to my friends too for their support throughout my

research.

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TABLE OF CONTENTS

ABSTRACT .................................................................................................................................... iii

ACKNOWLEDGEMENT .............................................................................................................. iv

TABLE OF FIGURES ................................................................................................................... vii

Chapter 1 .......................................................................................................................................... 1

1.1 INTRODUCTION ..................................................................................................................... 1

1.2 Solid state CdTe PV cell ............................................................................................................ 4

1.3 Review of Literature .................................................................................................................. 5

1.4 CdTe fabrication methods .......................................................................................................... 7

1.5 The Pourbaix diagram of CdTe- H2O system. ........................................................................... 9

Chapter 2 ........................................................................................................................................ 11

2.1 Objectives ................................................................................................................................ 11

2.2 Hypothesis ............................................................................................................................... 12

2.3 Electrodeposition ..................................................................................................................... 12

2.4 Photoelectrochemistry .............................................................................................................. 13

Conduction Band ................................................................................................................... 14

Valence band .......................................................................................................................... 14

Band gap ................................................................................................................................ 15

2.5 Significance of the study .......................................................................................................... 16

Chapter 3 ........................................................................................................................................ 17

3.1 Materials and experimental methods ....................................................................................... 17

3.2 Cyclic voltammetry .................................................................................................................. 23

3.3 Xenon Lamp ............................................................................................................................ 24

vi

3.4 Powder X-Ray diffraction ........................................................................................................ 26

3.5 Scanning electron microscopy (SEM) ..................................................................................... 27

Working principle of SEM ..................................................................................................... 27

3.6 Profilometer ............................................................................................................................. 28

Working principle of profilometer ......................................................................................... 29

3.7 Photoluminescence experiment ............................................................................................... 29

Chapter 4 ........................................................................................................................................ 30

4.1 Results and discussion ............................................................................................................. 30

4.1 Electrodeposition trials ............................................................................................................ 37

4.2 Scanning Electron Microscopy Studies ................................................................................... 41

4.3 Powder X-Ray Diffraction (PXRD) ......................................................................................... 53

4.4 Film thickness/profilometry ..................................................................................................... 54

Chapter 5 ........................................................................................................................................ 55

5.1 Conclusion and future work ..................................................................................................... 55

References ...................................................................................................................................... 57

vii

TABLE OF FIGURES

Figure 1: Illustration of some of potential renewable energy ............................................. 1

Figure 2: Schematic diagram showing electrode liquid junction Schottky barrier ............. 5

Figure 3: Atomic structure of CdTe .................................................................................... 7

Figure 4: Schematic diagram showing basic regions in Pourbaix diagram. ....................... 9

Figure 5: Equilibrium potential–pH (Pourbaix) diagram of the CdTe–H2O system ........ 10

Figure 6: Schematic diagram showing our proposed electrode ........................................ 11

Figure 7: Schematic diagram of a photoelectrochemical. ................................................. 14

Figure 8: Schematic diagram showing bands in a semiconductor. ................................... 15

Figure 9: Image of a three-electrode setup cell. ................................................................ 19

Figure 10: Schematic diagram showing steps on how to make SS304 electrode ............. 22

Figure 11: Stainless Steel 304 electrodes .......................................................................... 22

Figure 12: Setup experiment for electrodeposition ........................................................... 24

Figure 13: Cyclic voltammogram of CdTe under Xenon lamp ........................................ 25

Figure 14: Image of Xenon lamp ...................................................................................... 25

Figure 15: Image of Powder XRD .................................................................................... 26

Figure 16: Schematic diagram of profilometer ................................................................. 28

Figure 17: Cyclic voltammogram for 0.5 M CdSO4. 50 mV/s sweep rate, SS 304

electrode, Ag/AgCl reference electrode, pH 1.8 H2SO4, under argon gas. ....................... 30

Figure 18: Cyclic voltammogram for Te (1×10-4 TeO2 in pH 1.8 H2SO4 on SS 304

electrode, Ag/AgCl reference electrode at 80 ºC, 85 ºC and 90 ºC scan rate 50 mV/s

respectively) ...................................................................................................................... 31

viii

Figure 19: Cyclic voltammogram of Te (-0.455V, 0.01V; at 80 ºC, 85 ºC and 90 ºC; scan

rate 50 mV/s)..................................................................................................................... 32

Figure 20: Temperature dependence cyclic voltammogram of CdTe deposition on bare

SS 304, 0.5 M CdSO4, 1×10-4 TeO2 in pH 1.8 H2SO4 (-0.455V, 0.01V; at 80 ºC, 85 ºC

and 90 ºC; scan rate 50 mV/s) ........................................................................................... 33

Figure 21: Cyclic voltammogram of CdTe deposition onto a Te/stainless steel 304

substrate 0.5 M CdSO4, 1×10-4 M TeO2, pH 1.8 H2SO4, Ar atmosphere’ Ag/AgCl

reference electrode at (80 ºC, 85 ºC and 90 ºC; scan rate 50 mV/s ). ............................... 34

Figure 22: Cyclic voltammogram for sweep rate dependence of the photoelectrolysis on

SS 304 in darkness, Ag/AgCl ref. electrode……………………………………………..35

Figure 23: Sweep rate dependence for cyclic voltammogram of CdTe film on Te/SS 304

in the light using xenon lamp…………………………………………………………...36

Figure 24: Electrodeposition of Te at 80º C, for 480 s………………………………….37

Figure 25: Electrodeposition of CdTe on at 25º C, -0.45 V applied potential…………..38

Figure 26: Electrodeposition of CdTe at 80º C, -0.45 V………………………………...39

Figure 27: Effect of Pt deposition on CdTe/Te/SS304 at 80º C, -0.45 V………………..39

Figure 28: Cyclic voltammetry of Pt/CdTe/Te/SS 304…………………………………..39

Figure 29: SEM images of Cd film developed by electrodeposition ((a) 1000× and (b) 50

× respectively) ............................................................................................................... 42

Figure 30: SEM images of Te film developed by electrodeposition ((a) 50×and (b) 500×

respectively) ...................................................................................................................... 42

Figure 31: SEM images of unannealed CdTe film developed by electrodeposition ((a)

500×and (b) 1000× respectively ....................................................................................... 43

ix

Figure 32: SEM image of unannealed CdTe film electrodeposited under Ar (2500×). ... 43

Figure 33: SEM images of annealed CdTe film at 350oC (3500×, 3000× and 1400×

respectively) ...................................................................................................................... 44

Figure 34: EDX peaks and image showing elemental composition of unannealed CdTe at

2500× ................................................................................................................................ 45

Figure 35: EDX peaks and image showing elemental composition of annealed CdTe at

1000× ................................................................................................................................ 46

Figure 36: EDX image for blank SS 304 substrate at 1000× ............................................ 47

Figure 37: EDX image for unannealed CdTe powder scraped off of stainless steel 304

substrate at 1000× ............................................................................................................. 48

Figure 38: EDX image for annealed CdTe powder scraped off a stainless steel substrate at

(1000× magnification) ...................................................................................................... 49

Figure 39: PXRD image for CdTe deposited on SS 304 .................................................. 50

Figure 40: Spectrum of profilometer showing CdTe film thickness ................................ 51

1

Chapter 1

1.1 INTRODUCTION

The energy we receive from the sun is abundant and free from any pollutants.

This energy can be put into use as thermal, chemical, or electrical processes after

converting it. Northern Africa, the Middle East, and the United States of America are

known to have some of the richest resources of solar energy worldwide. For the past six

decades, consistent progress has been made in developing technologies to harness

electricity from solar radiation.1

Currently modern technology is being used to harness this energy for many

purposes such as generation of electricity, which can be used for heating water for

domestic, commercial, or industrial purposes. 1 Due to population growth and rise in

standards of living, the global energy demand is increasing rapidly. The illustration

shown below shows some of the potential renewable energy that can be used.

Figure 1: Illustration of some of potential renewable energy

2

In recent years, research towards sustainable and renewable energy has increased

significantly. Cadmium telluride (CdTe) photovoltaic cells work as a semiconductor that

converts energy of sunlight into DC (direct current) electricity. CdTe is an ideal absorber

material for high-efficiency low cost thin film polycrystalline solar cells. 2

Photovoltaic (PV) cells are important since they work as an integral part of solar-

electric energy systems, thus increasingly making them important as alternative sources

of utility power. Suitable semiconductors such as CdTe must therefore collect the

available photon energies, which are ideally equivalent to the band gap energy of the p-

type semiconductor.

The cadmium telluride semiconductor is well known as a stable photo-electrode,

with a band gap which is well suited to absorb most of solar spectrum. CdTe has a band

gap of 1.45eV, thus making it close to the ideal value for an efficient photovoltaic

conversion. High absorption of light from the sun and chemical stability are found for

CdTe. Thin film cadmium telluride (CdTe) is now regarded as a leading material for the

development of cost-effective photovoltaics.3

As compared to crystalline silicon, which was originally used to manufacture PV

cells, cadmium telluride is known to have several advantages. Thin CdTe films can be

made extremely thin, about a few micrometers, far thinner than a typical Si layer on order

of 100µm. Cadmium telluride is more efficient as compared to Si, especially at elevated

temperatures and low levels of illumination. Besides that, CdTe PV cells require less

energy for their production and due to its high optical absorption coefficient, the

compound is an attractive material for optoelectronics, gamma ray detector, and infrared

3

detector besides its PEC application. For this case, high quality growth of cadmium

telluride fine crystalline film is important because of its potential applications in

semiconducting devices, photovoltaic optoelectronic devices, radiation detectors, laser

materials, thermoelectric devices, solar energy converters, Videocon devices, sensors and

nano devices.4

Nevertheless, there are environmental concerns due to cadmium in CdTe PV

cells. Cadmium is a heavy metal that is known to be toxic, just like mercury and lead.

However, proponents of CdTe technology report that cadmium ion is insoluble in water,

so that Cd2+ should not pose any hazard to the groundwater. Furthermore, CdTe will not

vaporize in a fire. Cadmium is a harmful element, but once incorporated into PV

modules; its effects are reduced, so it does not cause a risk to the public.

A large number of deposition methods have been applied to CdTe, resulting in

high-quality films or layers leading to high-efficiency cells in economic production on

the other hand. Close spaced sublimation (CSS) is the most popular technique for

obtaining highest efficiencies; Spraying or screen printing are techniques with high

economic potential. It is remarkable that the highest efficiency CdTe PV devices are

fabricated from polycrystalline rather than single crystalline CdTe. A probable reason is

that grain boundaries enhance the collection of photogenerated minority carriers.5 In this

work, investigation of the electrode process of co-reduction of Cd2+ and HTeO2+ ions in

aqueous sulfuric acid was done to prepare cadmium telluride films by electrodeposition.

Characterization of cadmium telluride films electrodeposited on a stainless-steel substrate

was done by use of Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray

(EDX) analysis.1

4

1.2 Solid state CdTe PV cell

One of the major challenges of CdTe solar cell research is the achievement of a

high hole density in the CdTe layer by means of controlled acceptor doping. CdTe grown

under Te rich conditions is mostly reported to be p-type and has a low resistivity. In this

case, from the research that has been performed, it is reported that utilizing a contact

material with the proper work function provides a low barrier height at the contact.

Heavy doping of the semiconductor adjacent to the contact promotes tunneling and adds

recombination centers to the semiconductor adjacent to the contact. Stainless steel 304 is

comprised of high percentage of iron, which is known to have a work function of 4.47eV.

One method of forming a tellurium contact is by electrodepositing it from a

solution containing Te ion. During tellurium layer formation, cadmium diffuses out of the

CdTe adjacent to the growing tellurium film, leaving CdTe with an excess of tellurium.

At the same time, tellurium may diffuse into this region aided by the cadmium vacancy

density, providing extrinsic p-type doping. This layer forms the actual contact or acts to

lower the barrier height. 11, 12

In general, as-prepared CdTe films fabricated by various techniques have high

electrical resistivity (107–109cm) and are slightly p-type doped due to the formation of

Cd vacancies in the CdTe lattice acting as acceptor centers.11, 22

In overall, photovoltaic cell is made up of a substrate, front contact, buffer layer,

absorber layer (p-type) and back contact. The substrate can be either metal, glass or

polymer foil. The substrate should be mechanically stable, have a matching thermal

expansion coefficient with the other deposited layers and be inert.

5

Figure 2: Schematic diagram showing electrode liquid junction Schottky barrier

1.3 Review of Literature

Cadmium telluride (CdTe) belongs to the II-VI family. It is a chalcogenide

semiconductor; it has one electropositive element (Cd) and one chalcogen element

(belongs to group 16, Te). The photovoltaic (PV) effect was discovered in 1839 by

Edmond Becquerel. The phenomenon can be explained by the origin of its name photo

from the Greek word phos, which means light and voltaic which means electrical. In

other words, the photovoltaic effects are about electricity generation from light

illumination. For a long time, it remained a scientific phenomenon with few device

applications. Research on CdTe was reported in the 1950s 6. According to Xavier, (2004),

research into CdTe/CdS junctions started in 1970s. He observed that the CdTe band gap

is almost a perfect match to the distribution of photons in the solar spectrum in terms of

6

conversion to electricity since it has a value of ~1.5 eV. CdTe was used as a p-type and

matched together to an n-type such as n-CdS semiconductor. The cell was prepared

through the process of top and bottom addition contacts method. Top and bottom addition

refers to transferring the materials to both surfaces of the substrate. In the 1960s, GE was

the leader in CdS/CdTe cell efficiencies. By using the close spaced sublimation (CSS)

technique, Kodak made the first 10% cells and the first multi-cell devices.6

The cathodic electrodeposition of CdTe for photovoltaic applications was initiated

by Monosolar, Inc. in 1976 and their subcontractor, the University of Southern

California.3 Nevertheless, CdTe cells did achieve more than 15% efficiency until 1992,

when a buffer layer was added to the tin conducting oxide (TCO)/CdS/CdTe stack and

the CdS layer was thinned to admit more light. Moreover, it is reported that in the early

1990s, some other upcoming companies got varied results. The British Petroleum

company (BP Solar) in the late 1990s did a demonstration project that yielded an

efficiency conversion greater than 10%.7 Golden Photon held the record for a short time

after fabricating the best CdTe module which was measured at NREL to be 7.7%

efficient, by use of a spray deposition technique. Of late a fruitful deposition of CdTe

and comprehensive material characterization was carried out using CdCl2 and

Cd(NO3)2 precursors.7,8

7

Figure 3: Atomic structure of CdTe

1.4 CdTe fabrication methods

This study will involve the fabrication of electrodes. According to Dharmadasa, 9

preparations of semiconductor electrodes have been accomplished by a wide variety of

methods that include vapor deposition, spraying, screen printing, thermal evaporation,

electrodeposition, radio-frequency (rf) sputtering, electron beam evaporation, laser

ablation, sol-gel films and chemical bath techniques and closed space sublimation.

Thermal evaporation in vacuum is often used because it offers many possibilities to

modify the deposition conditions and study the effect of preparation conditions on the

physical properties of the respective films. It has been well established that in this

preparation method the process parameters like substrate temperature, source

temperature, and deposition frequency (rate) change the quality and physical properties of

the CdTe films.

The results of many research efforts have demonstrated that CSS is one of several

techniques with large area manufacturing potential due to its high throughput and

efficient material utilization.10

8

A conducting glass substrate is used as the semiconductor support. This method is

known to produce stable films, albeit at high materials and energy cost. Among these

techniques, electrodeposition is popular due to its simplicity and the economical

technology used. In our case, the electrodeposition technique was used.

Electrodeposition and electroless deposition have been practical alternatives.

Electroless plating has drawbacks; for instance, the bath (electrolyte) must be heated.

This causes evaporation, adding to energy cost. The bath has a limited life time. The film

that has been plated may too off the substrate. Some of the pits of the material may lead

to formation of metal particles in the solution due to improper rinsing. Thus, the bath

plates these particles, eventually destroying the bath in the process. Very thorough

filtration must be used. Another challenge is that most electroless baths are useful only

for coating metal with a thin layer. Hence electrical plating may be required to thicken

the metal film. Moreover, some metals do not easily formulate through the electroless

method.

Electrodeposition provides numerous advantages. It produces a film growth of

any shape and size, allows ambient temperature deposition, and film properties can be

easily altered based on the bath composition, applied voltage and deposition period. It is

among the common techniques used to prepare polycrystalline CdTe films. It is an

inexpensive procedure which can be used for fabrication of thin-film semiconductors. It

provides large area coatings on a material and more, so it does well on the complicated

surfaces.

9

1.5 The Pourbaix diagram of CdTe- H2O system.

Pourbaix diagram also known as Potential-pH diagram, has been named after the

founder, Marcel Pourbaix (1963), a Belgium electrochemist and corrosion scientist. The

diagrams represent the stability of a metal as a function of potential and pH. Pourbaix

diagrams are commonly used to assess the effects of pH, oxidation potentials, and

activities of potential-determining ions on the chemical processes of rock and soil

formation. The main functions of the Pourbaix diagram are: (i) it gives various reactions

directions, (ii) it can be used to make a basis for the estimation of the corrosion; and (ii) it

shows which pH region and potential changes will reduce or prevent corrosion. As shown

in the figure below, there are four regions in the diagram corresponding to oxidizing

(acidic), oxidizing (alkaline), reducing (acidic) and reducing (alkaline) environments.

Figure 4: Schematic diagram showing basic regions in Pourbaix diagram.

10

As shown below in Figure 5, Pourbaix diagram was used as a guide to know

exactly the stable pH of the CdTe bath condition. From the figure, CdTe is shown to have

stability limits at lines 1 and 4 (lower limit) and 5, 11 and 12 (upper limit). This is an

indication that cadmium telluride is thermodynamically stable at this point. In acidic,

neutral and basic solution at potential above the lower stability limits, hydrogen is

released at the cathode 23. CdTe reduces to cadmium and hydrogen telluride at pH values

of 2.0 and cathodic potential greater than -1.5 V. Similarly, it corrodes to Cd2+and Te at

smaller cathodic potential less than 0.1 V. it is observed that most of the work on CdTe

deposition was carried out in acidic media, since alkaline bath were often found to yield

poorly adhering deposit.

Figure 5: Equilibrium potential–pH (Pourbaix) diagram of the CdTe–H2O system

Marcel Pourbaix (1963)

11

Chapter 2

2.1 Objectives

1. To use electrodeposition method as a means of producing CdTe thin films on

stainless steel 304, brass and copper.

2. To apply them in a photoelectrochemical water-splitting scheme to produce hydrogen

using Xe lamp radiation as a source of simulated solar energy.

Figure 6: Schematic diagram showing our proposed electrode

12

2.2 Hypothesis

The pros and cons of semiconductors for energy generation are reviewed with

their operation principles and physical efficiency limits. The main materials currently

used or investigated and the associated fabrication methods for making CdTe are

described. Semiconductors fabricated from silicon dominate in terms of production as

compared to that of CdTe. Recent developments suggest that thin-film CdTe is a

promising candidate for future photovoltaics.

Our main purpose in this work is to develop low cost and efficient CdTe films

using stainless steel 304(SS 304) substrates, which will serve as photoelectrodes in a

photoelectrochemical cell, through which hydrogen is produced by photoelectrolytic

water splitting. This thesis is a continuation of Dr. Stephen Rhoden’s work, 5 and clearly

shows that modified electrodes must have more than enhanced kinetics. The intent is to

make thin films of CdTe on the stainless steel 304 substrates. This approach was

reported, 40 years ago by Honda and Fujishima.6 It involved the use of TiO2 as the

semiconductor electrode for water photolysis.

2.3 Electrodeposition

This is the deposition of a substance on an electrode by the action of an external

source (electricity). It is a technique that uses no vacuum and it gives good control over

the properties of the thin films through the influence of parameters such as deposition

potential, bath temperature, pH, and deposition time and electrolytes concentration. It is a

metal deposition process in which positively charged metal ions (M+) from the solution

are reduced and deposited on the surface of a negatively charged electrode (cathode) by

the passage of electric current. During electrodeposition, the substrate is immersed into

13

an aqueous solution, and polarized negative of the reduction potential, for this case

CdSO4 and TeO2, holding a temperature of around 80-90 °C.

2.4 Photoelectrochemistry

The electrodeposited CdTe on SS 304 substrate was used as the working electrode in

photo electrochemical cells. The counter electrode was a platinum foil or strip. The

principle of photoelectrochemical water splitting is based on light conversion into

electricity within a cell in which two electrodes immersed in a bath or electrolyte, of

which at least one is made of a semiconductor exposed to light and able to absorb light.

The resulting photovoltage is then used to drive water electrolysis. In the

photoelectrochemistry (PEC) of semiconductors, the electrochemical response of the

system is studied under the illumination of light. Incident photons generate electron-hole

pairs in the semiconductor. These energized electrons get excited to the conduction band

and corresponding holes are created in the valence band. Further movement and

recombination lifetime of the minority and the majority carriers depend on the type of

semiconductor and its physical properties.

14

Figure 7: Schematic diagram of a photoelectrochemical.

Photovoltaic cells can have either direct or indirect bandgaps depending on the positions

of the valence band maximum and the conduction band minimum. CdTe is an example of

direct bandgap photovoltaic cell that can absorb light more easier because an electron in

the valence band can be promoted directly to the conduction bandgap without a change in

momentum.

Conduction Band

This is the energy band whose electrons can be used to conduct current. Conduction

bands are made up of lowest energy empty orbitals.

Valence band

This is the highest energy band below the conduction band that is occupied. Valence

bands are made up of highest energy filled orbitals.

15

Band gap

The energy separation between the valence and conduction band in such a picture is

called the band gap.

Figure 8: Schematic diagram showing bands in a semiconductor.

Fujishima and Honda first showed that illumination of n-type semiconductors

greatly reduced the voltage required to split water. To split water, a minimum potential

difference of 1.23 V must be established between the anode and cathode. In a typical

photoelectrochemical cell, the anode is an n-type semiconductor and the cathode would

be a platinum foil, but for our case, the cathode is a p-type semiconductor and the anode

is a platinum foil. The semiconductor electrode is irradiated to promote an electron from

valence band to conduction band, generating current through the external circuit. The

electron in the conduction band results in reduction of hydrogen ion of water at the

semiconductor, while holes in the valence band will meet with electrons from external

circuit to oxidize water to generate oxygen at the platinum counter electrode.

16

Since previous research by Dr. Rhoden (2012) showed that the idea behind CdTe

is for redox chemistry to be performed on water, thus splitting it and providing hydrogen

gas for use as stored solar energy as shown by the equation:

2H2O 4H+ (aq) + 4e- + O2 (g) [E0 = -1.23 V]

4H+ (aq) + 4e- 2H2 (g) [E0 = 0.00 V]

Net: -1.23V

A semiconductor capable of spontaneous water splitting must have a conduction

band energy higher and a valence band energy lower than the reduction potential for

hydrogen ion to H2, Ered (H2/H+) and the oxidation potential for O2 evolution from water

Eox (OH-/O2), respectively. To drive the transfer of each electron in the process, the

semiconductor must supply an electro potential energy equivalent of at least 1.23 V. In

this work, the electrodeposition will be carried out in aqueous solution using a 3-

electrode potentiostated system.

2.5 Significance of the study

This study will be important to an analytical student because using CdTe PV cells to

split water and produce hydrogen has many applications:

For example, highly pure hydrogen can be obtained in requisite amounts which can be

used to store energy;

It can be useful in space shuttle and space missions.

17

It can be used in fuel cells and automobile applications.

Hydrogen is used in petroleum refining processes.

Chapter 3

3.1 Materials and experimental methods

Our research objective was to prepare suitable photovoltaic electrodes, which can

be assembled in a photochemical cell for hydrogen production by water splitting.

Equipment used for this experiment are Potentiostat (Model 273), hot plate, two wires

and alligator clips. All the chemicals and substrates used in this work were purchased

from Alfa Aesar and Fisher Chemicals. The anode and cathode clipped by an alligator

clip were then connected to a Potentiostat using a wire. The CdTe thin films were

potentiostatically electrodeposited on stainless steel 304 substrates using aqueous

solutions containing CdSO4 and TeO2 solution.

Solutions to be used for initial voltammetric studies were prepared using

deionized water to make different concentrations as follows:

0.01042g CdSO4 50ml 0.5 M CdSO4

0.007979g TeO2 50ml 1×10-4 M TeO2

0.2777ml concentrated H2SO4 50ml 1×10-2 M H2SO4

18

For the sulfuric acid, 0.28 ml of concentrated stock solution was taken and diluted to

make a volume of 50 ml as shown above. This solution was used to adjust the pH to 1.8.

All lab equipment and some chemicals were already available in the laboratory.

The stainless steel 304 substrates obtained were cut to 2.6cm x 0.6cm rectangles before

any initial processing was performed. Trace metal grade hydrochloric acid (HCl), 99+ %

w/w cadmium sulfate (CdSO4), Chloroplatinic acid hexahydrate (H2PtCl6) and 99.99+%

tellurium oxide (TeO2) were obtained from Alfa Aesar. 96% w/w sulfuric acid (H2SO4)

was acquired from Fisher Scientific. For most of the work, deionized water was used for

rinsing, cleaning, and making all solutions. The solutions were kept for a day before

being used.

The stainless-steel squares were cleaned with detergent, followed by acetone,

methanol and then rinsed with deionized water and finally dried carefully. It was then

submerged and stirred in a 10% v/v HCl bath, followed by washing with deionized water

and consequent drying of surface by compressed argon gas. Deionized water was

generally used for cleaning, rinsing, and making solutions.

The electrolytes were prepared by dissolving cadmium and tellurium precursors in

acidified water. The electrodeposition of CdTe semiconductor thin films is effected

usually from aqueous acidic solutions. The concentrations varied from 1 mM to 0.5 M.

The precursors of Cd2+ and HTeO2+ ions were anhydrous CdSO4 and TeO2, respectively.

Semiconductor electrodeposition testing was done using a PAR 273A Potentiostat

in a conventional single compartment cell, with a three-electrode setup, consisting of a

stainless-steel 304 working electrode, a silver/silver chloride reference electrode (0.22V

19

vs NHE- Normal Hydrogen Electrode) and a platinum strip electrode (110 mm2 surface

area). The electrode at which the reaction of interest occurs is called working electrode.

For this case, in the three-electrode system, the potential of the working electrode was

measured relative to the reference electrode, which itself has a known and stable

electrode potential. Thus, the potential on the working electrode is precisely

controlled.7,11 This has the advantage of producing layers with controlled stoichiometry

since the effects of changes in certain deposition parameters such as stirring can be

minimized. Figure 9 below is a setup cell which was used in electrodeposition.

Figure 9: Image of a three-electrode setup cell.

20

The SS 304 sample was immersed in 1M HCl solution for 1 minute to strip away

any residual oxide layers on the surface, followed by a rinse in deionized water and blow-

drying, after which the samples were inserted in the cell ready for deposition. Before the

addition of TeO2, soaking of the electrode in CdCl2 solution was done. The cyclic

voltammograms of CdTe were recorded using SS 304 substrate as the working electrode

(cathode) to determine the possible deposition potential of CdTe. The pH of the

electrolyte was adjusted to 1.8 with H2SO4. Some of the experiments were carried out at

25oC. First, an electrolyte containing only Cd2+ in aqueous sulfuric acid was examined at

80oC by cyclic voltammetry on a stationary stainless steel at various scan rates.

Stainless steel electrodes were prepared by covering one side of the substrate with

an easily removable rubber cement, allowing it to air-dry overnight. A solution of 0.5 M

CdSO4 and 1 mM TeO2 was made and the pH adjusted to 1.8 with H2SO4 and heated to a

constant temperature of 80 oC. A water bath was used to equilibrate the temperature at

80oC. Heating will have the benefit of improving the crystallinity of the semiconductors

deposited.3, 12

All solutions were kept at constant temperature using water baths and were

moderately stirred with a heating plate. Before electrodeposition was performed, all

solutions were purged with argon gas for at least 10 minutes to remove dissolved air.

CdTe was deposited directly to HCl-etched stainless-steel by applying voltages varying

between -0.455V and -0.75V at various times. The deposition potentials of films were

determined from cyclic voltammetry data of the deposition solution. To improve CdTe

crystallization, a final annealing of the electrode was done for 1 hour at 350oC under

21

argon atmosphere. The CdCl2 treatment at 350 °C in the presence of oxygen is known to

improve the cell efficiency.

The electrodes for photoelectrochemical testing were completed by attaching a

wire to the backside plate with silver epoxy, air-drying for 2 hours and finally covering

the entire attached electrode wire and backside with silicone. Photoelectrochemical

testing was done in stirred electrolytes, using a three-neck round bottom flask. The

cathodes were oriented vertically and had an exposed area of 2-4cm2, 12. A parallel Pt foil

was used as anode in the cell and electrolysis under controlled potential conditions was

conducted. Analytical electron microscopy images were taken using a JEOL JIB 4500

Focused Ion Beam/Scanning Electron Microscope (FIB/SEM) to show morphology with

an energy dispersive X-ray (EDX) detector for elemental analysis. Voltammograms were

obtained by performing cyclic voltammetry using the potentiostat. In the present

research, cyclic voltammetry was the primary technique.

22

Figure 10: Schematic diagram showing steps on how to make SS304 electrode

Figure 11: Stainless Steel 304 electrodes

23

3.2 Cyclic voltammetry

Cyclic voltammetry (CV) is an electrochemical technique that is mostly used in

characterization of oxidation and reduction systems and for performing electroanalytical

studies. It is used to provide quick information about the number of redox states of the

electroactive species. It is also used to give qualitative information concerning the

stability of the oxidation states of the elements and kinetics of the electrons.3, 16 CV is an

extremely powerful and popular electrochemical technique that is used to characterize the

oxidation-reduction properties of compounds and to examine the mechanisms of redox

reactions. The CV experiment involves linearly changing the potential of the working

electrode comparative to a reference electrode and measuring the resulting current. The

potential changes linearly from an initial value to some arbitrary final value, and is then

reversed back to the initial value, which completes the cycle. The electrodeposition setup

involved the use of a three-electrode system (working electrode (the cathode), counter

electrode (the anode), and the reference electrode) and most of the work done so far on

electrodeposition of semiconductors in general has been based on the three-electrode

system. Cyclic voltammograms were recorded for separate deposition electrolytes at

85oC to study the deposition mechanism of Cd and Te to form CdTe. The working,

counter and reference electrodes were placed within proximity to each other in a closed

cell, under an inert atmosphere. Typical CV parameters used were 0 V for both initial and

end potential, two vertex potentials 0.01 V and −7.5 V, scan rate from 50-200 mV/s. The

extrema at which reversal takes place (in this case, -7.5 V and 0.01 V) are called

switching potentials. The range of the initial scan may be either negative or positive,

depending on the composition of the sample. A scan in the direction of more negative

24

potentials is termed a forward scan while one in the opposite direction is called a reverse

scan.

Figure 12: Setup experiment for electrodeposition

3.3 Xenon Lamp

A xenon arc lamp was used in this research as a source of simulated sunlight to

regenerate the adsorbent. It works as a lamp that discharge a gas, an electric light that

produces light by passing electricity through ionized xenon gas at high pressure. It gives

out a bright light that closely mimics natural light from the sun. figure 13 below shows

cyclic voltammogram of CdTe done at different sweep rates. Newport 500W and 1000W

xenon arc lamps were used in this work. Shown are voltammograms of CdTe obtained in

the dark and in the light by using a xenon lamp.

26

3.4 Powder X-Ray diffraction

Powder X-ray diffraction is known to be used for the study of crystal structures

and for finding atomic spacing. It is based on the constructive interference of

monochromatic X-rays which are generated by an X-ray tube. Filtration is done to

produce monochromatic radiation; the beam is aligned through collimation, and finally it

is directed to the sample. Constructive interference is produced when the lattice spacing

in the sample enables the difference in the pathlength between the diffracted beam and

the incident rays to be an integer multiple of the wavelength. The figure below shows an

image of Power X-ray diffractometer at YSU which was used for characterization.12 The

crystallinity of thin films was measured by X-ray diffraction using Cu Kα radiation (λ =

1.5404).

Bragg’s equation is given as:

2dsinθ = nλ

Figure 15: Image of Powder XRD

27

3.5 Scanning electron microscopy (SEM)

A scanning electron microscope is a type of electron microscope that can produce

high resolution images of a sample by scanning it with a focused beam of electrons.

SEM–EDX technique was used to provide detailed imaging information about the

morphology and surface texture of individual particles, as well as elemental composition

of samples. In our study, the sample was cut into approximately 3 mm × 4 mm pieces and

mounted in the SEM-EDS chamber. The electrons interact with atoms in the sample,

producing secondary electron signals that can be detected and that contain information

about the sample's surface topography and composition. The electron beam is generally

scanned in a raster scan pattern, and the beam's position is combined with the detected

signal to produce an image. An SEM with sufficient beam voltage and sample enables the

difference in pathlength between the diffracted beam conductivity can achieve resolution

better than 1 nm.

Working principle of SEM

SEM mainly uses a beam of electrons which are ejected or emitted terminally from a

tungsten or lanthanum hexaboride (Lab6) cathode and accelerated towards the anode.

Tungsten is used simply because it has high melting point and a low vapor pressure,

hence allowing it to be heated for electron emission. The electron beam has an energy

that ranges from a few hundred eV to 100 KeV and is focused by a condenser lens into a

beam with a very fine focal spot. The beam goes through scanning coils in the objective

lens, which deflects the beam horizontally and vertically so that it scans in a raster

fashion over a rectangular area of the sample surface. As the electron beam interacts with

the sample, the electrons lose energy by repeated scattering and absorption within a

28

teardrop-shaped volume of the specimen known as the interaction volume, which extends

from less than 100 nm to around 5 micrometers into the surface. The size of the

interaction volume depends on the electrons’ landing energy and the atomic number of

the sample. This results in the emission of electrons and electromagnetic radiation, which

lead to detection of the image.13

3.6 Profilometer

It is an instrument that is used to measure surface's profile or roughness of a

sample. It has a diamond stylus that physically touches the surface. The KLA-Tencor

AlpaStep Development Series Stylus Profiler model was used.

It was used to give the difference between the high and low point of sample surface in

nanometers.15

Figure 16: Schematic diagram of profilometer

29

Working principle of profilometer

The KLA-Tencor is computerized and has high sensitivity surface profiler that measures

step height, roughness and waviness in a variety of applications on the surfaces. It

measures precision heights from under 10 angstroms to as large as 1.2 millimeters. It

does this by incorporating a new optical deflection height measurement mechanism and

magneto static force control system that results in a low force and low inertia stylus

assembly. The scan parameters such as speed, length, sampling rate and force to applied

are set. By using high force it gives high resolution of the sample .The scan gives a graph

of the data. Both height and width can be measured moving R and M scale parameters to

level the graph of the data.

3.7 Photoluminescence experiment

Photoluminescence (PL) is an important physical phenomenon used to

characterize semiconductors which depicts the electronic structure of the materials while

possibly revealing other important material features. In brief, PL occurs when a

photovoltaic cell absorbs light of energy higher than the bandgap. This results in the

creation of electrons and holes in excess of their thermal equilibrium concentrations.

After thermal relaxation, the excess electrons and holes recombine with a consequence of

photon emission (radiative recombination) with lower energy than the excitation photons.

The recombination is accomplished either directly (such as band-to-band recombination –

electrons in the conduction band recombining with holes in the valence band) or more

frequently, involving one or more intermediate states such as bound excitons and

impurities or defects with an energy level inside the forbidden gap.

30

Chapter 4

4.1 Results and discussion

A potentiostat was used to obtain voltammograms of CdTe. An electrode was

immersed in an ionic solution containing an electroactive species, or analyte, and the

electro potential on the electrode was swept while current flow was monitored. There was

no current flow until oxidation or reduction at the electrode occurred. After the voltage

over a set range in one direction, the direction is reversed and swept back to the original

potential. The cycle could be repeated as often as desired. The figures below show the

current versus volts curves for the deposition of CdTe onto the pretreated stainless-steel

samples. The cathodic processes on stainless-steel at different temperatures in sulfuric

acid containing Cd2+ and HTeO2+ were examined in detail by cyclic voltammetry.

Figure 17: Cyclic voltammogram for 0.5 M CdSO4. 50 mV/s sweep rate, SS 304

electrode, Ag/AgCl reference electrode, pH 1.8 H2SO4, under argon gas.

-1.00 -0.75 -0.50 -0.25 0 0.25-0.015

-0.010

-0.005

0

0.005

0.010

E (Volts)

I (Am

ps/cm

2 )

cd3.cor

31

For initial characterization of Cd electrodeposition was performed to identify Faradaic

activity by applying a voltage of -0.75 V. the ionic liquid was purged with argon and the

voltammetric experiments were performed using a 273 PAR potentiostat driven by

CorrWare. With a negative voltage limit equal to -0.5 V, the onset of a cathodic was

observed (Figure 17). Thus, it was discovered that the supposed cathodic solvent limit

probed in the figure above was the onset of a voltammetric wave with peak at -0.5 V and

not more than -0.75 V.

Figure 18: Cyclic voltammogram for Te (1×10-4 TeO2 in pH 1.8 H2SO4 on SS 304

electrode, Ag/AgCl reference electrode at 80 ºC, 85 ºC and 90 ºC scan rate 50 mV/s

respectively)

-0.50 -0.25 0 0.25-0.0025

0

0.0025

0.0050

E (Volts)

I (Am

ps/c

m2 )

te31.corte90.cortel86.cor

32

Cyclic voltammetry was performed to show temperature effect on the

electrodeposition of tellurium. With a negative voltage limit equal to -0.45 V, the onset of

a cathodic reduction as observed above (Figure 17) started at voltage of about -0.2 V.

Thus, it was discovered that the supposed cathodic solvent limit probed in the figure

above was the onset of a voltammetric wave where we expect tellurium reduction.

Figure 19: Cyclic voltammogram of Te (-0.455V, 0.01V; at 80 ºC, 85 ºC and 90 ºC; scan

rate 50 mV/s)

The voltage scale in this figure indicates the cathode potential measured with respect to

an Ag/AgCl reference electrode. The first rise in the current at around -0.2 V illustrates

the formation of tellurium on the cathode.

-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1-0.0010

-0.0005

0

0.0005

0.0010

E (Volts)

I (A

mps

/cm

2 )

te90.corte85.corTe80.cor

33

Figure 20: Temperature dependence cyclic voltammogram of CdTe deposition on bare

SS 304, 0.5 M CdSO4, 1×10-4 TeO2 in pH 1.8 H2SO4 (-0.455V, 0.01V; at 80 ºC, 85 ºC

and 90 ºC; scan rate 50 mV/s)

In order to determine the electrodeposition mechanism and the codeposition

potential for CdTe precursor, cyclic voltammetry measurements were performed on bare

stainless steel substrate. It was observed in the Figure 20 above that by sweeping

voltammogram to the cathodic limit on a bare stainless steel substrate started a wave that

showed up at -0.25 V.

This indicated that CdTe deposition took place for cathode potentials between -

0.2 V and -0.5 V. The temperature was varied as seen in the three cycles above. It was

-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1-0.0015

-0.0010

-0.0005

0

0.0005

E (Volts)

I (A

mps

/cm

2 )

cdte90.corcdte85.corcdte80.cor

34

observed that increase in temperature resulted in increase of currents too. From the waves

observed it was noted that electrodeposition of CdTe on a bare stainless steel substrate

does not yield a good result of reduction. A light thin film of grey deposition was

observed on the substrate.

Figure 21: Cyclic voltammogram of CdTe deposition onto a Te/stainless steel 304

substrate 0.5 M CdSO4, 1×10-4 M TeO2, pH 1.8 H2SO4, Ar atmosphere’ Ag/AgCl

reference electrode at (80 ºC, 85 ºC and 90 ºC; scan rate 50 mV/s ).

It was observed in the Figure 21 above that the onset of cathodic reduction is

slightly lower than the previous one. Stirring and heating was done as well. The

difference seen is a result of depositing of CdTe on a Te/SS 304 substrate. This

voltammogram gave a reversible wave. The temperature too was varied as previous and

35

still reversible waves were obtained. It was noted that cadmium undergoes underpotential

deposition (UPD) as shown in Figure 21 above. UPD is a surface-limited phenomenon

that results in a limited deposition of atoms at a potential positive of its normal Eº. CdTe

was codeposited, in that both elements were deposited at a constant potential from the

same solution. It is noted that cadmium has a strong preference to be to deposited onto a

tellurium layer as opposed to a bare stainless steel substrate. For this case it is observed

that cadmium UPD peak shifted towards a more positive potential as compared to that of

a bare substrate. Hence in conclusion it shows that the interaction of CdTe/Te/SS 304 and

CdTe/SS 304 is not the same.

Figure 22: Cyclic voltammogram for sweep rate dependence of the photoelectrolysis on

SS 304 in darkness, Ag/AgCl ref. electrode

-0.5 -0.4 -0.3 -0.2 -0.1 0 0.1-0.0005

0

0.0005

0.0010

E (Volts)

I (A

mps

/cm

2 )

[email protected]@[email protected]@dark200.cor

36

Figures 22 pH 1.8 H2SO4 was used as control. A flat baseline of about 0.25 MmAa for

electrodeposition of other substances

Figure 23: Sweep rate dependence for cyclic voltammogram of CdTe film on Te/SS 304

in the light using xenon lamp.

Sweeping the potential at different rates resulted in an increase of current as the

sweep rates were increased. Each curve in the figure above has the same form and it was

observed that the total current increases with increasing sweep rate. It was noted that the

linear sweep voltammogram took longer to record as the sweep rate was decreased.

37

4.1 Electrodeposition trials

The oxygen evolution reaction, which is oxidation of water, is one of the

important reactions that occur at the anode. It is coupled with other important reactions

such as hydrogen evolution in water splitting. The factors such as mechanism of electrode

reaction, kinetics, thermodynamics, effective surface area, conductivity and chemical and

mechanical stability were put into consideration for an overall performance.

It was first necessary to deposit a thin underlayer of Te. The potentiostat was set

to sweep negative from 0.001 V at 50 mV/s and hold at -0.45 V for 480 s for Te and 600

s for CdTe. Stirring and heating was performed while argon gas was bubbled in the cell

for purging. The current verses time profiles from Te deposition are shown below. We

were able to observe dark grey deposition on the substrate.

Figure 24: Electrodeposition of Te at 80º C, for 480 s

0 100 200 300 400 500-0.0020

-0.0015

-0.0010

-0.0005

0

Time (Sec)

I (A

mps

/cm

2 )

TeO2oct17.cor

38

Figure 25: Electrodeposition of CdTe on at 25º C, -0.45 V applied potential

Figure 25 above shows the electrodeposition of CdTe under illumination and in

darkness. It was observed that there was a little increase in current by shining light on the

SS 304 PV cell as in the red line. The xenon lamp was used in the experiment as a source

of light in order to simulate sunlight.

0 250 500 750-0.0050

-0.0025

0

0.0025

Time (Sec)

I (Am

ps/c

m2 )

cdtedarktial1.corcdtexetrial1.cor

39

Figure 26: Electrodeposition of CdTe at 80º C, -0.45 V

In Figure 26 above, to allow for longer cathodic electrolysis times, a sweep-and-hold was

performed. The working electrode was swept from 0 to -0.45 V at 50 mV/s. At that point,

argon purging was started. The experiment was stopped at 480 s or 8 minutes in tellurium

deposition, and 600 s in other trials. The chronoamperometric data is as shown in the

Figure 26 above. For longer cathodic electrolysis times in order to deposit CdTe, time

was extended to 7200 s.

0 250 500 750-0.004

-0.003

-0.002

-0.001

0

Time (Sec)

I (A

mps

/cm

2 )

cdtexetrial1.cor

40

Figure 27: Effect of Pt deposition on CdTe/Te/SS304 at 80º C, -0.45 V

Figure 28: Cyclic voltammetry of Pt/CdTe/Te/SS 304

0 25 50 75 100-0.04

-0.03

-0.02

-0.01

0

Time (Sec)

I (Am

ps/c

m2 )

pt.cdte.te.ss304.100

-0.75 -0.50 -0.25 0-0.100

-0.075

-0.050

-0.025

0

0.025

E (Volts)

I (Am

ps/cm

2 )

CDTEdark150.corCdTeXelamp.corpt.dte.light.cor

41

The effect of platinum on electrodeposited CdTe was tested with just some few

seconds remaining before electrolysis was stopped as seen in Figure 27 above. It was

observed that there was an increase of bubbles of hydrogen gas. This was done by

sprinkling little amount of chloroplatinic acid into the electrolyte. This is so because

platinization of the CdTe photovoltaic cell makes its kinetics of H2 gas evolution

increase. It was noted that there was an increase in current due to platinum activation, as

seen in the hydrogen peak displayed in Figure 28. Performing photoelectrochemistry both

in the dark and light did not show any change in current as seen in Figure 28 above. It

was concluded that platinization activated the surface of the electrodes.

4.2 Scanning Electron Microscopy Studies

The substrates at their various stages of development were examined using

Energy Dispersive Analysis of X-rays (EDX) data while SEM was done on the samples.

Measurements were performed on the samples deposited AT -0.455V verses Ag/AgCl.

The following figure shows scanning electron microscopy top views of the films at

different magnification.

42

(a) (b)

Figure 22: SEM images of Cd film developed by electrodeposition ((a) 1000× and (b) 50

× respectively)

(a) (b)

Figure 23: SEM images of Te film developed by electrodeposition ((a) 50×and (b) 500×

respectively)

43

(a) (b)

Figure 24: SEM images of unannealed CdTe film developed by electrodeposition ((a)

500×and (b) 1000× respectively

Figure 25: SEM image of unannealed CdTe film electrodeposited under Ar (2500×).

44

Figure 26: SEM images of annealed CdTe film at 350oC (3500×, 3000× and 1400×

respectively)

SEM images at different magnifications are shown in the figures above from various spot

areas of the sample to show the film quality in different regions. As seen above some of

the SEM images show cracks on the surface. The cracks can arise from incomplete

coalescence of the CdTe grains during deposition or be due to defects in the underlying

surface or may have been developed during annealing process. The films were allowed to

cool to room temperature to avoid building up of stress in the films.

Below is the EDX data showing elemental composition of the sample. The spectra

of unannealed sample shows the present of Cd and Te while in the annealed sample it is

observed that there is addition of chromium and Iron.16 These additional elements found

45

are known to be the characteristics of the stainless steel 304.

Figure 27: EDX peaks and image showing elemental composition of unannealed CdTe at

2500×

46

Figure 28: EDX peaks and image showing elemental composition of annealed CdTe at

1000×

47

Figure 29: EDX image for blank SS 304 substrate at 1000×

From Figure 36 above the morphological characteristics of a bare stainless steel was

studied. It was observed that stainless steel 304 is majorly comprised of iron and

chromium elements.

48

Figure 30: EDX image for unannealed CdTe powder scraped off of stainless steel 304

substrate at 1000×

In the Figure 37 above, unannealed commercial CdTe powder were characterized in

SEM/EDX. Epoxy glue was used to make contact between the support material and the

powder. It was observed from the studies that there was presence of oxygen and chloride.

The possibility of their presence could have been as a result of epoxy.

49

Figure 31: EDX image for annealed CdTe powder scraped off a stainless steel substrate at

(1000× magnification)

Commercial CdTe powder was taken to a furnace to be annealed at 350 ºC. After

annealing the epoxy was used to make a contact of CdTe powder and the support

material. As compared to Figure 37 above there was no change in the percentage of

elemental composition. Fortunately, from the results seen in Figures 37 and 38 above this

phenomenon worked to our advantage as the sample surface becomes Te rich (more p-

type), which is suitable to form a good ohmic contact.

50

Figure 32: PXRD image for CdTe deposited on SS 304

51

Figure 33: Spectrum of profilometer showing CdTe film thickness

Profilometry is a surface topography technique where a stylus is mechanically

drawn across a sample. Features are measured by deflections of a laser off the top

surface of the stylus into a photodiode array.

The voltammograms from the electrodeposition experiments clearly show that

reactions are occurring and show reduction peaks. 17. Figures 1 to 5 show cyclic

voltammograms of aqueous 0.5M CdSO4; 1mM TeO2 and CdTe mixture during the

forward and reverse cycles. The scanning rate was fixed at 5mV.s-1 and the electrolyte

temperature was kept at 80oC. It is observed that Cd2+ deposits at -0.45V with respect to

52

Ag/AgCl reference electrode. The noise in the cyclic voltammogram at high voltages

could be due hydrogen gas evolution. From the voltammogram observed in this work, the

suitable voltage range for stoichiometric CdTe layer is observed from -0.3 to -0.45 V.

A cathodic limiting current or a shoulder located at -0.75V and an associated

anodic peak around 0.01V can be observed. The data recorded comparable results

repeatedly. It is proposed that the cathodic reaction is a two-electron transfer with Cd

film formation:

Cd2+ + 2e- → Cd

Deposition of CdTe cathodically has been shown to vary by various conditions

like deposition voltage, pH, electrolyte concentration, and temperature and stir rate.15,18

The electrodeposition of CdTe in an acidic solution involves the reduction of HTeO2+ to

Te, which in turn reacts with Cd0 to produce CdTe. The reactions expected during

deposition occur as follows:

HTeO2+

(aq) + 3H+ (aq) +4e- → Te (s) + 2H2O (l)

Cd2+ (aq) + HTeO2

+ (aq) + 3H+ + 6e-→CdTe (s) + 2H2O (l)

53

From the SEM images it was observed that annealing temperature of 350 ͦC which

enhanced nucleation led to formation of large grains18. By treating with CdCl2 the CdTe

layers grown in this work showed a compact grain growth without gaps. This suggests

that CdCl2 treated CdTe produced from chloride precursor has a better quality as seen in

the SEM images19,20.

The thickness of the electrodeposited CdTe layer on the SS 304 was calculated

experimentally by using a profilometer. The highest reading that was seen in the

profilometry was 75000 Armstrong as shown in the Figure 40 above. From the

measurements made an approximate of 7.5 micrometers of film thickness was obtained.

4.3 Powder X-Ray Diffraction (PXRD)

The X-ray diffraction studies were carried out on as-deposited CdTe layers and

typical results are shown in Figure 35 below. In this study, CdTe thin films were

electrodeposited at a voltage of 450 mV vs Ag/AgCl. According to Figure, the graph

shows the presence of the noticeable peak of CdTe at 2θ = 24.15° corresponding to the

(111) cubic phase. However, the pattern displays six diffraction peaks values at 47.2,

63.1, 72 and 77.5, which correspond to the diffraction lines produced by the (311), (331),

(422) and (511) crystalline planes of cubic CdTe, respectively. The large peaks shown

correspond to iron.

CdTe samples were annealed under an ambience of argon. For CdCl2 presoaking

is well known that its treatment usually improves the crystallinity of CdTe up to a certain

temperature. 21

54

4.4 Film thickness/profilometry

The film thickness was calculated as per the formula;

For Te, the deposition current is at most 1 mA. Therefore,

μm=(i.t.fw)/(nFQA)

=[current(C/s)(time(s)(FW(g/mol)]/ [F(coul/eq)(4 eq/mol)(density, g/cm3)(A,cm2)]

=[(1E-3)(8 min)(60 s/min)(127.6 g/mol)(1E-4 μm/cm)] ÷

[(96485C/eq)(4 eq/mol)(6.25g/cm3)(2cm2)]

=0.127 μm; or 127 nm.

Calculating for film thickness CdTe resulted in about 8 μm as shown in calculations

below.

=[current(C/s)(time(s)(FW(g/mol)]/ [F(coul/eq)(eq/mol)(density, g/cm3)(A,cm2)]

=[(1E-3)(120 mins)(60 s/min)(240 g/mol)(1E-4μm/cm)] ÷

[(96485 C/eq)(2 eq/mol)(5.85 g/cm) (2cm2)]

=7.6 μm.

55

This is so impressive as far as thin film thickness is necessary in making CdTe

photovoltaic cell. The results from the profilometer as seen in Figure 40 above shows a

film thickness of approximately 0.75 μm

In conclusion, we have obtained information from XRD and SEM

characterization techniques indicating the growth of a new film on the substrate and the

formation of CdTe.

Chapter 5

5.1 Conclusion and future work

In this work it was demonstrated that the electrodeposition of CdTe on SS-304 is

feasible. Thin films of CdTe were grown by applying a potential of -0.45 V verses

Ag/AgCl for 120 min to give a film thickness of approximately 7-10 µm. Structural and

optical properties of these films were investigated by using various characterization

techniques. Powder XRD analysis showed that CdTe films are cubic polycrystalline and

has a preferred orientation in (111) direction. Furthermore, annealing improves

crystallinity in the CdTe films. Energy Dispersive X-ray Spectroscopy (EDS) was used to

acquire detailed imaging information about the morphology and surface texture, as well

as elemental composition of the film.

Thin films of CdTe have been successfully grown through an electrodeposition

method as seen from the SEM images. The visual appearances, surface morphology,

56

elemental and optical properties of CdTe layers were grown on the stainless steel 304

substrates are shown.

It was noted that platinization of the CdTe photovoltaic cell made the kinetics of

H2 gas evolution to increase by nearly two orders of magnitude. The voltammetric results

presented here and other relevant information from the literature lead to the following

conclusions. It is noted that cadmium has a strong preference to be deposited onto a

tellurium layer as opposed to a bare stainless steel substrate. By performing

electrodeposition it was observed that cadmium UPD peak shifted towards a more

negative potential in comparison to that of a bare substrate. Use X-ray photoelectron

spectroscopy to measure elemental composition of Cd and Te. Increasing substrate

temperature can passivate grain boundaries and improve grain integrity effectively, along

with decreased radiative recombination.

57

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